1. Field of the Invention
The present invention relates generally to chlorinated hydroxyrubber, a method for making chlorinated hydroxyrubber, and environmentally friendly low volatile organic content coatings comprising chlorinated hydroxyrubber.
2. Background
Chlorination of cis-polyisoprene is a well established industrial process to produce chlorinated rubber. See, e.g., British Patent No. 381,038 (1859) and U.S. Pat. No. 1,234,381 (1912). Chlorinated rubber has long been used in surface coatings to provide abrasion resistance, chemical and water resistance, flame retardance, durability and toughness. It is one of the best anticorrosive binders widely used in hostile environments such as marine coatings, swimming pool coatings and traffic paints.
Chlorinated rubber, however, is a non-convertible or nonreactive binder. That is, chlorinated rubber does not react with other ingredients in the coating composition. Thus its applications have been limited to solvent based coatings of high volatile organic content (VOC). Such coatings are cured exclusively by solvent evaporation, which causes air pollution. Although still allowed, high VOC coatings have been largely replaced by new technologies such as water borne, high solids, and powder coatings in response to stringent legislative regulatory requirements.
Chlorinated rubber coatings suffer from a further problem of poor solvent resistance due to lack of crosslinking between the chlorinated rubber binder and the solvents or diluents used in the coating compositions.
These problems associated with chlorinated rubber coatings have existed since chlorinated rubber was first utilized in the coatings industry approximately fifty years ago. They have become more serious since the Environmental Protection Agency began in 1967 to regulate VOC levels in coatings and they will become even more serious as more and more stringent environmental regulations take effect in the future.
A need therefore exists for a functionalized chlorinated rubber that will act as a convertible binder, i.e. a reactive binder, in coating applications, thereby reducing VOC levels, and that will impart improved solvent resistance to coating formulations.
Functionalization, or chemical modification, of chlorinated rubber has been a topic of extensive studies in recent years. Most of the studies have used direct modifications of chlorinated rubber, such as free radical initialized graft copolymerization with a variety of functional acrylate or styrene monomers. Direct modifications of chlorinated rubber are limited, however, due to its chemical inertness and thermal instability. Moreover, unwanted side reactions such as polycondensation and elimination are unavoidable.
Hydroxylation of rubber via saponification of the reaction adduct of rubber and haloacetic acids has been recently reported. Y. H. Kim and A. Pandya, Macromolecules, 24, 6505-11 (1991). However, the commercial value of the product, partially hydroxylated rubber, also known as hydroxyrubber, was not assessed, nor were coating applications mentioned, since hydroxyrubber does not possess properties necessary for coating grade materials.